Wheels for mounting automotive tires thereon are manufactured by joining a disk formed as a circular plate and a wheel rim formed as a hollow cylindrical body to each other, by welding or the like. Such a wheel is referred to as a two-piece wheel.
According to a method of manufacturing a wheel rim, as disclosed in Japanese Laid-Open Patent Publication No. 9-206951 and Japanese Laid-Open Patent Publication No. 10-129204, a plate having an elongate rectangular shape is curved and opposite ends thereof are brought into abutment against each other, producing a hollow cylindrical body, and the abutting opposite ends (abutting regions) are resistance-welded by means of a so-called resistance butt welding process. According to Japanese Laid-Open Patent Publication No. 62-107832, it is proposed to form a hollow cylindrical body in the same manner as described above, and to join the abutting regions by MIG welding or TIG welding.
When abutting regions are welded by the welding processes disclosed in Japanese Laid-Open Patent Publication No. 9-206951, Japanese Laid-Open Patent Publication No. 10-129204, and Japanese Laid-Open Patent Publication No. 62-107832, metal in the vicinity of the weld tends to rise in a swelling formation. Since swelling makes the welded wheel rim low in product quality, the welded wheel rim needs to be finished to grind off such swelling. Accordingly, wheel rims cannot be manufactured efficiently using the disclosed welding processes.
Friction stir welding may be used to weld abutting regions, without producing swelling on the welded assembly, and hence without the need for finishing the welded assembly. According to a friction stir welding process, a friction stir welding tool is rotated, and a probe on the tip end of the friction stir welding tool is plunged into abutting regions of the end faces that are to be welded. Frictional heat is generated in the vicinity of the abutting regions, causing a plastic flow of the material at the workpiece ends including the end faces thereof, so as to join the end faces to each other.
Because the probe is pressed into the abutting regions to be welded during the friction stir welding process, the abutting regions are liable to become spaced apart, tending to develop a clearance therebetween. When such a clearance is formed, the welded strength is lowered, thereby producing a welding defect.
In order to avoid the above shortcomings of the friction stir welding process, a process for friction-stir-welding sheets, to press the end faces of the sheets along a direction in which the rotating tool is displaced has been proposed, thereby preventing the sheets from becoming spaced apart, as disclosed in Japanese Laid-Open Patent Publication No. 10-193139. However, although the proposed process is effective for joining sheets together, it is not applicable to manufacturing a hollow cylindrical body, such as a wheel rim or the like.
According to the friction stir welding process, if the probe is plunged such that its central axis is aligned with the joint between abutting end faces, then unjoined regions may remain in a probe operation surface and an opposite surface (reverse side) in the abutting regions. When such unjoined regions remain, they reduce the bonding strength of the weld. If the bonding strength is excessively small, then the welded assembly may possibly start to crack away from the weld when plastic machining is performed on the workpiece. Japanese Patent No. 2808943 proposes a process for avoiding welding defects produced by laser welding using a filler. However, the proposed process cannot be used to avoid welding defects produced during a friction stir welding process, since a filler is not employed in the friction stir welding process.
According to one solution to the problem of remaining unjoined regions, it is customary to keep the tip end of the probe spaced from the reverse side of the workpiece, by a distance (gap) of 0.1 mm or less. However, providing such a small gap is not easy, and it takes a long time to form the gap between the tip end of the probe and the reverse side of the workpieces.
When a plate-like workpiece is curved and its opposite ends are brought into abutment against each other, producing a hollow cylindrical body to manufacture a wheel rim, one of the opposite ends possibly may overlap with the other end. A hollow cylindrical body with overlapping ends cannot be joined by friction stir welding.
The above drawback can be eliminated by spacing the end faces slightly apart from each other and then bringing them back into abutment with each other, without causing overlapping, after the diameter of the hollow cylindrical body is slightly increased. However, carrying out such a process is tedious and time-consuming, and tends to lower production efficiency of the friction stir welding process.
Another problem, even if opposite ends of the hollow cylindrical body are prevented from overlapping with each other, is that the hollow cylindrical body may have an elliptical cross-sectional shape, rather than a true circular cross-sectional shape. In this case, since the hollow cylindrical body does not have a true circular cross-sectional shape, the welded workpiece cannot produce a usable product, or stated otherwise, product yield is lowered.
If the hollow cylindrical body has an elliptical cross-sectional shape that is elongate horizontally, then as shown in FIG. 23, end faces 1, 2 abut against each other while being positionally displaced, so as to be oriented toward a center of the cross-sectional shape while being spread away from each other.